Static exciters are widely used for feeding the field winding of electric generators.
US2007/0296275 discloses (with reference to FIG. 1 of this document) a static exciter 1 having an exciter transformer 2 that is connected at one side to a busbar 3 of a generator 4 (i.e. to the output of the generator) and at the other side to a rectifier 5 that converts the AC voltage fed from the exciter transformer 2 into a DC voltage.
The rectifier 5 is typically driven by an AVR (Automatic Voltage Regulator) 6 that is connected to the busbar 3 via a voltage transformer 7.
The DC side of the rectifier 5 is connected in series with the field winding 8 of the generator 4 and a forward biased diode 9.
The forward biased diode 9 is connected in parallel with a capacitor bank 10; moreover a switch 11 is provided between the capacitor bank 10 and the diode 9; such switch 11 can be closed to connect the capacitor bank 10 in series with the rectifier 5.
During normal operation the switch 11 is open; the AC voltage from the exciter transformer 2 is converted into a DC voltage and is fed to the field winding 8 because the diode 9 is forward biased and conducts.
In case a disturbance at the grid occurs (for example due to a short circuits in the transmission line) the voltage of the grid drops and, therefore, also the voltage of the busbar 3 and the voltage at the lower voltage side of the voltage transformer 7 drops.
When the voltage at the lower voltage side of the voltage transformer 7 drops below a prefixed voltage value, the switch 11 is closed such that the diode 9 is reverse biased (and does not conduct anymore) and the capacitor bank 10 is connected in series with the rectifier 5 and field winding 8.
This causes a considerably higher field voltage to be applied to the field winding 8 and guarantees the stability of the generator/grid system.
Nevertheless, once a disturbance occurs, the capacitor bank 10 is fully discharged, therefore if a plurality of disturbances occurs the system is not able to cope with them.
In fact (FIG. 1), even if the static exciter 1 is provided with a charging device 12 (such as a battery) that keeps the capacitor bank 10 charged, the charging device dimension is usually small and it takes minutes to hours for it to recharge the capacitor bank 10 to an acceptable charge level.